v] 



PYRENOMYCETES 



141 



only on the somewhat incomplete evidence that they give rise to ascogenous 

 hyphae and on the basis of their resemblance to the sexual branches of other 

 Ascomycetes. There is a very similar coiled hypha in certain species of 

 Eurotium which is certainly a functional archicarp. Comparison may also 

 be made with the female branch of Ascodesmis nigricans and with that of 

 the Erysiphaceae. 



(ii) The second type of pyrenomycetous initial organ (fig. 102) may 



Fig. 102. Polystigma rubrumJ^C'', mature 

 archicarp, x 800 ; after Blackmail and 

 Welsforcl. 



Fig. 103. Xylaria polymorfha (Pers.) Grev.; 

 archicarp embedded in stroma, x 1000. 



readily be derived from the first. It occurs in forms where the perithecium is 

 immersed either in the substratum or in a stroma, and its essential character 

 is the prolongation of the tip of the archicarp to form a trichogyne-like 

 organ. The appearance of this structure is associated with the development 

 of spermatia in spermogonia. Archicarps of the type in question are found 

 in Polystigma among the Hypocreales and in Gnomonia, Poronia and 

 Mycosphaerella among the Sphaeriales. In all these genera, however, the 

 trichogyne appears to be merely vestigial ; in Polystigma it never reaches 

 the exterior of the host-leaf, in Gnomonia its connection with the coiled 

 oogonial region is doubtful and in Mycosphaerella and Poronia it degenerates 

 early. In Polystigma the ascogenous hyphae arise from vegetative cells and 

 not from the archicarp and it is at least possible that the same is the case 

 in the other genera named. A comparison is obvious between the archicarps 

 of these forms and those of several Lichens and of such Discomycetes as 



